Such double-wall pipes are used in district heating pipes, for example. The diameter of the plastics outer tube in such cases is generally between 75 mm and 1,200 mm, although larger diameters are also possible.
The pipes consist of an inner tube that conveys media when in use and is usually made from steel, although other materials can also be used. For thermal insulation, the medium pipe is surrounded by a heat insulating layer for insulation purposes, said layer usually being a rigid foam. A plastics outer tube representing a mechanical protective jacket and often consisting of polyethylene forms the outer seal.
The adhesive bond of such a sandwich system is very important for the purpose of pipe statics in the case of hot pipes laid underground, as all thermal forces are transmitted in a frictional manner. In such cases, it is particularly important that the insulation provided for heat insulation purposes fulfills its function fully over a long period of time.
However, in practice, it has been found that such heat insulating layers, in particular rigid foam insulation, specifically those made from polyurethane foam, are subject to an increasing ageing process, especially if the media are at high temperatures in excess of approximately 120° C. This not only has an adverse effect on the heat insulation value, but also on the mechanical adhesive bond.
The ageing behavior is due in particular to the gases penetrating the plastics outer tube, particularly oxygen. The penetrating oxygen leads to oxidation processes within the insulation which cause the properties of the insulation to deteriorate. The CO2 bonded in the pores is diffused outwards and replaced by nitrogen from the atmosphere, leading in turn to a deterioration of the insulating effect.
The above-mentioned problems occur in particular in the rigid polyurethane foams used as insulation. Decomposition due to oxidation leads to destruction of the “cell windows” in the polyurethane foam. This results in a reduction in the compressive strength and in the ability of the foam to withstand shear stress until mechanical failure of the system.
WO 2004/003423 A1 describes an insulated pipe comprising one or more inner tubes and an insulating foam surrounding the inner tube. The pipe has a layer that regulates diffusion, said layer being applied to the outer face of the foam. Furthermore, the pipe may comprise an outer tube made from a durable material, preferably polyethylene. The diffusion-regulating layer may be arranged between the polyethylene outer tube and the polyurethane foam or on the outer face of the polyethylene outer tube.
If the diffusion-regulating layer is arranged on the outer face of the outer tube, there is a risk that this layer may be damaged or destroyed by mechanical effects. Arranging the diffusion-regulating layer between the outer tube and the polyurethane foam also has disadvantages because the inner tube (steel tube) or the spacer guiding the inner tube comes into contact with the diffusion-regulating layer during manufacture of the pipe when inserting the inner tube into the outer tube by manual means or by machine, and may damage or destroy said diffusion-regulating layer if it is only very thin and designed such that it can be mechanically damaged.
Pipes comprising a stainless steel tube as a medium pipe, an insulating layer made from polyisocyanurate foam (PIR), a gas permeation barrier layer and a low-density polyethylene (LDPE) jacket are already known.
DE 20 2010 018 026 U1 relates to a plastics outer tube for a plastics jacketed pipe which comprises a medium pipe, particularly made from steel, a plastics outer tube acting as a mechanical protective jacket, and insulation filling the annular gap between the medium pipe and the outer tube to provide thermal insulation. In this case the plastics outer tube is designed as a multilayer tube and comprises a plastics gas permeation barrier layer consisting of ethylene vinyl alcohol copolymer (EVOH) in the inside thereof, integrated in the outer tube. An inner and an outer adhesion-promoting layer are provided adjacently to the gas permeation barrier layer. The plastics outer tube consists of an inner polyethylene layer, an inner adhesion-promoting layer of the gas permeation barrier layer made from EVOH that is adjacent to said inner polyethylene layer, an outer adhesion-promoting layer that is adjacent to said inner adhesion-promoting layer, and an outer polyethylene layer. The gas diffusion barrier layer is 100 μm thick, whereas the entire plastics outer tube is 3 mm thick.
Flexible composite pipes for district heating pipes are also already known that are formed of an inner and an outer corrugated pipe having polyurethane foam heat insulation arranged between said pipes and are able to be rolled up on reels.
It is therefore already known to provide a gas permeation barrier layer on such pipes or plastics outer tubes in order to prevent the outside atmosphere, and in particular oxygen, coming into contact with the inner foam layer and causing or accelerating the ageing thereof.
EP 1 471 299 A1 describes a thermally insulated pipe comprising at least one inner tube, a heat insulating layer, and an outer jacket made from thermoplastic material, a thermoformable plastics film being provided in said pipe between the heat insulating layer and the outer jacket. The plastics film has a core layer, for example made of polyethylene terephthalate having a material thickness of 14 μm, and two boundary layers bonded by adhesion-promoting layers.
EP 0 569 681 A1, EP 0 836 042 A1 and US 2008/0128042 A1 also describe the use of polyethylene terephthalate as a material for gas permeation barrier layers in multilayer tube configurations.
EP 0 960 723 B1 also relates to a composite element, for example for the packaging industry, having a layer sequence consisting of polyethylene, aluminum and polyethylene terephthalate, and EP 1 394 457 B1 relates to a hose having low permeation, comprising at least one coolant barrier layer and a rubber layer or a resin layer, polyethylene terephthalates preferably being used as polyester resins.